Automotive signal lamps must have specific colors defined by international traffic regulations. For example, automotive signalling functions like tail, stop, or fog lighting require red lamps. In order to obtain the red color as defined by international traffic regulations, prior-art lamps are usually coated with a red varnish or lacquer. A disadvantage of the prior-art lamps is that the lacquer or varnish may degrade and may eventually peel off from the wall of the lamp envelope. Actually, these red coatings usually consist of vulnerable constituents that have a relative low thermal resistance. Another disadvantage of the prior-art lamps is that additional steps are necessary during the manufacturing process of the lamp for cleaning and varnishing the lamp envelope. The manufacturing cost is increased by these additional steps.